Wiring Loom


The wiring of my GT6 was pretty shot, so I decided to make a new one from scratch!! In the end this probably turned our more expensive than buying a new loom from Autosparks (when taking into account investment in tools), but had some key advantages: I could customise it, and get satisfaction from doing it myself. Also I could recoup the cost of the necessary tools by selling them on eBay when the job is finished (although I may be tempted to keep them!!)

I started by making the mini-loom which feeds the heated rear window and interior light, then the main loom from the 8-pin bullet type connector under the dashboard on the passenger side to all the rear lights, heated rear window and interior light. I couldn't get a replacement connector so I reused the original by cuttung the wires and joining the new wires to the old. Then, I intend to tackle the main loom covering the dashboard, engine compartment and front lights, horns, etc.

First I had to make some basic decisions:-

1. What type of cable to use, and
2. What type of connectors to use.
3. How to fuse the circuits.
4. Whether to introduce more relays.

 Two types of cable are available for this application, original single core PVC and thin wall single core PVC cable. According to Vehicle Wiring Products in Ilkeston, Derbyshire, from whom I bought all my supplies thin wall single core PVC cable is "a high performance cable which as the name suggests has thinner insulation. It is multi-stranded copper cable which is insulated with thinner hard grade PVC which offers good resistance to abrasion and cut through. The cable also carries a higher load compared to the standard auto cable. Gives savings in weight and volume when used in large harnesses. This cable is now original equipment on most new vehicles". I decided to go with this, as it would make the loom smaller (in diameter) make it easier to get mutiple cables through tight spaces and take less room generally. Below is a table of the available single core PVC cables with detailed specifications:

Ref No. of strands Strand dia (mm) Strand area (mm2) Total area (mm2) Cable ID (mm) Cable OD (mm) Wall thickness (mm) Max Load (amps) Typical Applications
16/0.20/0.5 16 0.20 0.03 0.50 0.80 1.60 0.40 11 Side & tail lights, alarms, reversing & rear fog lamps, electric aerial, horns, general wiring.
32/0.20/1 32 0.20 0.03 1.01 1.13 2.00 0.43 16.5 Side & tail lights, alarms, reversing & rear fog lamps,
electric aerial, horns, general wiring.
28/0.30/2 28 0.30 0.07 1.98 1.59 2.70 0.56 25 Headlamps, fog and driving lamps, rear/front screen heater, wiper motors.
44/0.30/3 44 0.30 0.07 3.11 1.99 3.30 0.66 33 Charging cable for battery feed & ammeter.
56/0.30/4 56 0.30 0.07 3.96 2.24 3.70 0.73 39 Charging cable for battery feed & ammeter.
84/0.30/6 84 0.30 0.07 5.94 2.75 4.30 0.78 50 Charging cable for dynamo, alternator and ammeter circuits.
97/0.30/7 97 0.30 0.07 6.86 2.95 5.00 1.02 57 Charging cable for heavy duty alternator and ammeter circuits.
120/0.30/8.5 120 0.30 0.07 8.48 3.29 5.50 1.11 63 Charging cable for heavy duty alternator and ammeter circuits.
80/0.40/10 80 0.40 0.13 10.05 3.58 6.50 1.46 70 Charging cable for heavy duty alternator and ammeter circuits.

I decided not to use the popular coloured insulated terminals, as they are less secure, and I believe the factory used non-insulated terminals. This meant buying a set of crimping and other tools which were not cheap, but the end result should look more professional!

The original GT6 fusebox had 3 glass fuses, all rated at 35 amps. I decided to keep this but not actually use the fuses (for appearances sake) and to relocate the wires to a new fusebox located in the front n/s passenger compartment. This modular fusebox (made by MTA in Italy) would make it safer and easier to locate any faults by using a fuse for more circuits rather than just 3!

Relays also improve safety by moving the highe current of some components from the switches to the relays themselves. Currently the only relays in the original loom were in the starter solenoid, and a separate one for the horn. I decided to use 3 more, one for each of the headlights and one for the heated rear window.

My newly designed loom can now be seen by clicking here to see my pdf booklet. I will now do the following: 1) Adapt the headlights to take DRLs in the place of the pilot lights, 2) Remove the dashboard and the old wiring loom. One thing I need to check is howe the Hazard Switch works (by putting a multimeter across the terminals) to ensure it works with my new wirng layout, and 3) starting building the new loom in stages (as per the pages in the booklet) starting with the ignition circuit and installing the new fuse box. This will be fully documented in my blog. Rather than actually building a "loom" I intend to just fit the wires, circuit by circuit, and when a circuit is functioning correctly I will tidy up the wires and encase them in Easy Loomâ„¢ Split Wire Sleeve made by Design Engineering Inc. in Ohio, USA.

1. Ignition Circuit

This consists of the starter solenoid, the alternator, the ignition switch and the coil/distributor. The solenoid is the hub of the system - it receives the main feeds from the battery and alternator (which is connected to the solenoid, rather than directly to the battery). There is also a secondary feed from the B+ terminal of the alternator - this is the power supply which "energises" the alternator from the battery. This powers the alternator field coils so electricity can be generated as the alternator spins. Another connection operates the alternator warning light on the dashboard. The starter motor is energised by connecting the smaller terminal to the main terminal via the ignition switch. This connection also serves as a feed for the items powered when the ignition is switched on, which is routed through the fuse box for safety.

2. Lighting Circuit

This includes all external lighting as well as the instrument illumination which comes on with the sidelights. I will use three relays for the High Beam, Dip Beam and DRLs. This will route all the wires to the front of the bonnet, as well as behind the dashboard to the instrument illumination and master light switch. Two of the relays will be standard SPST (Single Pole Single Throw) used for the headlamps, and the other relay will be SPDT (Single Pole Double Throw) for the DRL. The thinking behind this is that the DRLs come on with the ignition, and extinguish when the lights are switched on. The interior roof light will be on a separate circuit and as it will be live without the ignition being on, so opening the doors lights up this light before the key is put into the ignition.